21 PRODUCT OF MEASURES 239 Let 9? be the restriction of the canonical mapping TT : R2 -» T2 to the set K of points (x, y) such that 0 <; x < I and 0 £y < 1, so that 9? is a continuous bijection of K onto T2. For t e T2 and (x, y) « g?" J(0, put p = \ir(2x, %(y + I)) Show that the mapping v : T2-»T2 ("baker's transformation") is continuous almost everywhere and that v o/=/0 w (where u is the mapping defined in (a)). We shall see later that u and v are ergodic (relative to v and /3, respectively) (Section 15.11, Problem 16). (c) Show that for the Bernoulli scheme B(/?0, ...,/?* _i), the entropy /z(«) (Section 13.9, Problem 28) is given by h(u) = -(po log A) H (Use the KolmogorofT-Sinai theorem, by starting with the partition a = where Aj is the set of (xn)H9Z such that XQ =j. Use Problem 9(a) and the definition of «, and remark that the set A,0 n u-^AjJ n • • - n u-'+^Aj^,) is the set of (.*„)„ «z such that XQ =j0 , xi =yl5 . . . , x,,-! = ^-i. The problem is then to calculate the sum £ . - -Pomo - mo + • • • + n\ti - i ™ n /^Q ! " * " Wfc _ \ I For this purpose, observe that Z — ~ - rWoPo0"1 mo + • • • + "'fc - 1 = " WQ I ' " ' MX, - i I up® Deduce from this calculation that the bijections u of the Bernoulli schemes B(J, J) and B(|, i, I) are not conjugate (Section 13.12, Problem 11). 19. Let X be a locally compact space, let ^ be a positive measure on X, and let /, g be two nonnegative /^-measurable functions on X. For each a > 0, let A« be the set of x e X such that f(x) > a. Suppose that : (i) /x(A«)< -f oo for all a>0; (ii) g e ^£(X, /x) for some/7 e ]1, + oo[; (iii) for each a > 0, /x(Aa) ^ — a J A Show that these conditions imply that/e ^f(X, ft) and that Np(/) g - 1 (Wiener's inequality). (Consider first the case where / is bounded and of compact support, and integrate from 0 to oo the inequality0 and ^pj = 1). On each factor